1. Field of the Invention
The invention is related to concentric casings and strings in wellheads wherein it is necessary to effect a seal between concentric members of the wellhead and is specifically directed to a seal system wherein the sealing members are activated via an external, non-invasive seal energizing system.
2. Discussion of the Prior Art
In oil and gas wells, it is conventional to pass a number of concentric tubes or casings down the well. An outermost casing is fixed in the ground, and the inner casings are each supported from the next outer casing by casing hangers which take the form of inter-engaging internal shoulders on the outer casing and external shoulders on the inner casing.
Typically, such casing hangers are fixed in position on each casing. There are however applications where a fixed position casing hanger is unsatisfactory, because the hang-off point of one casing on another may require to be adjusted. Such drilling wellheads have to accommodate a casing with an undetermined hang-off point, it has been known to use casing slip-type support mechanisms.
Wellheads are used in oil and gas drilling to suspend casing, seal the annulus between casing strings, and provide an interface with the BOP. The design of a wellhead is generally dependant upon the location of the wellhead and the characteristics of the well being drilled or produced. One specific type of wellhead is a unitized wellhead for platform or land applications.
Unitized wellheads are composed of several individual components, including a wellhead housing that is used to support a number of casing hangers and tubing hangers. The hangers support the weight of the casing and tubing, and pass loads back to the wellhead housing. Annulus seals seal the annular spaces between casing and tubing strings.
Conventional land or platform wellheads are either slip-type conventional wellheads or through-the-BOP multi-bowl wellheads.
Slip-type wellheads use casing slips to support casing strings. These slips are friction wedges that “grip” the top of a casing string and use slip teeth to bite into the casing. Wellheads of this type require higher-risk operations, as they require lifting the BOP to install casing slips and annulus seals. The seals that are used with slip-type casing hangers must be actively maintained throughout the field life of the well.
Multi-bowl type wellheads feature reduced-risk operations, as the BOP does not need to be lifted to set casing slips. Instead of using slips, a multi-bowl wellhead uses a fixed landing shoulder in the wellhead housing to support the first casing hanger. All other casing hangers are stacked on top of this initial casing hanger. The seals installed on multi-bowl wellheads can be more dependable than those installed in slip-type wellheads, but are still often unreliable, due to eccentricities in the casing hanger/wellhead alignment and unreliability in the seal setting mechanisms. As the initial load shoulder must support the weight of all casing strings and any loads due to test pressures, this load shoulder must intrude into the bore of the wellhead quite a bit. This can create an operational restriction that limits operations through this well.
Various sealing devices are known and employed in such wellheads. One example of a sealing assembly is shown and described in U.S. Pat. No. 4,913,469, wherein a wellhead slip and seal assembly includes a slip assembly with slips supported within a slip bowl and a seal assembly positioned above the slip assembly and interconnected thereto for supporting the slip assembly, the seal assembly includes two segments connected to form the seal ring and each of the segments includes arcuate elements embedded in a resilient material which forms an inner seal in an inner groove. The segments of the slip bowl include segments interconnected by toe nails and the seal ring includes pin and recess connection for connecting the two segments together.
It is also known from European Patent No. 0 251 595 to use an adjustable landing ring on a surface casing hanger to accommodate a space-out requirement when the casing is also landed in a surface wellhead.
More recently, and as shown and described in my U.S. Pat. Nos. 6,092,596 and 6,662,868, an external clamp for clamping two concentric tubes, typically two concentric tubes in an oil or gas well, has two axially movable tapered components which can be pulled over one another in an axial direction to provide a contraction of internal diameter which grips the smaller diameter tube.
Another example of a sealing system is shown and described in U.S. Pat. No. 5,031,695, wherein a well casing hanger with a wide temperature range seal element is energized by axial compression with a pre-determined initial portion of the casing hang load, the remaining portion of that hang load then being transferred to the wellhead or other surrounding well element without imposition on the seal element.
U.S. Pat. No. 6,488,084 shows and describes a casing hanger adapted for landing on a load shoulder in a wellhead to seal and support a string of casing. The casing hanger has a lower ring for landing on the load shoulder, the lower ring having an upward facing surface. A plurality of circumferentially spaced recesses are in the upward facing surface of the lower ring, each of the recesses having a base. A seal is located on the lower ring and has a plurality of holes that register with the recesses in the upward facing surface of the lower ring. A slip assembly bowl has a wedging surface that carries a plurality of slip members. The slip members grip the casing and cause the bowl to transmit downward forces from the casing to the seal to axially compress and energize the seal. Fasteners extend from the lower ring through apertures provided in the seal into threaded apertures provided in a downward facing surface of the bowl to secure the lower ring to the slip assembly but allow relative axial movement between the bowl and the lower ring. A plurality of substantially cylindrical stop members are located in the holes in the seal and in the recesses of the lower ring. The stop members are secured into threaded holes formed in the shoulder ring and contact the bases of the recesses to limit the compression of the seal to a predetermined amount.